Revert "travis: use gcc-5 instead of gcc(-4.8)"

Revert "travis: use gcc-5 instead of gcc(-4.8)"

This reverts commit 2b8221d66a8284042fc53c7bbb14bb9750e9137f.

Avoiding the use of GCC >= 5 stuff lets use go back to 4.8 simplifying
the travis setup again as well as reducing the backport requirements in
general.

This is possible because the std::get_time use requiring GCC >= 5 in
9febc2b238e1e322dce1f94ecbed46d595893b52 was replaced by handrolling it
in 1d742e01470bba27715a8191c50adde4b39c2f19, so the remaining uses are
just small conviniences we can do without.

Gbp-Dch: Ignore